Near-field magnetooptic microscopy uses an optical probe. The results obtained hitherto show a resolution of a few hundred nanometers.
Magnetic force microscopy (MFM) is used to probe the leakage fields near the surface of a system exhibiting magnetic order. This technique, based on the measurement of the force induced by the leakage field on a magnetized tip, reveals (under certain conditions) the domain structure. This is currently the reference technique for domain imaging in thin magnetic films with an optimum resolution of a few tens of nanometers.
More recently, images of the magnetism on the atomic resolution have been obtained by spin-polarized tunnel microscopy (SPSTM) using a magnetic tip. This technique relies on the fact that, under suitable polarization conditions of the tunnel junction, the intensity of the tunnel current depends on the parallel or antiparallel configuration of the respective moments of the tip and of the surface.
The two techniques—magnetic tip SPSTM and MFM—mentioned above have opened up an enormous range of possible ways of studying magnetism on the nanoscale. However, these techniques suffer from a limitation due to the use of a magnetic tip that interacts with the surface under study and may significantly modify the local magnetic properties thereof.
One possible application of the electron injector is in the imaging of magnetism using a GaAs tip. Several years ago it was proposed to inject spin-polarized electrons generated by light excitation in a semiconductor tip. This idea is contained in the patent of Alvarado et al., “Spin Polarized Scanning Tunneling Microscope”, European Patent 0 355 241, of 1990, one interesting aspect of which is described in FIG. 4. The electrons are spin-polarized if the light is circularly polarized (σ+ or σ− helicity) and if the light energy, greater than the bandgap energy Eg is furthermore less than the sum Eg+Δ where Δ is the spin-orbit interaction energy. For GaAs, the available energy window ranges from 1.42 eV to about 1.70 eV at ambient temperature. This operating mode has two advantages: firstly, the average spin of the injected electrons is controlled by the helicity of the light and changes sign when this helicity passes from σ+ to σ−. This makes it possible, by modulating the polarization of the light, to modulate the sign of the spin of the injected electrons. Thus, this is an independent measurement of the topography of the surface, by the average value of the contactless current, and the surface magnetism, by measuring the modulation of this current induced by the polarization modulation. Moreover, since the total magnetization of the photoelectrons is very low, the tip does not disturb the magnetism of the surface.
Owing to these advantages, several groups have attempted to apply this same idea, the light excitation taking place either from the side [see for example Prins et al. Phys. Rev. 53, 8105, (1996)], through the specimen, assumed to be transparent to the light. [W. Nabhan et al., Appl. Surf. Sci. 144-145, 570 (1999)]. The results obtained are not as convincing as those obtained with magnetic tips. In particular, strong parasitic effects, not associated with the magnetism, have been obtained. These effects mask the observation of the actual magnetic effects. These parasitic effects are attributed to the change in spin polarization during penetration of the light into the tip in the first case, and to the dichroism of the specimen in the second.
Another interesting related field is that of spin injection for spintronics and for quantum computing. The development of future components for spintronics and for quantum computing requires the injection of spin into semiconductor or metallic specimens, or into structures such as quantum dots. To achieve this injection without losing the spin, it is necessary to establish a barrier of controlled thickness between the injector and the system into which injection is to take place, which arrangement may be achieved more easily under tunnel injection conditions. Moreover, the injection into quantum dots requires the injector to be moved, which is why the tunnel imaging conditions are the most suitable.
Local probe techniques are envisioned for increasing the density of data storage. The company IBM has developed the “Millipede”, which consists of a matrix of cantilevers allowing both writing and reading. Going on from the promising results obtained, several alternative forms of storage systems using local probe techniques have been proposed (heat-induced storage, ferroelectric storage, etc.), but no local probe storage system seems to dominate for the moment.